In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, or laptops with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spècial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a user equipment has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, a base station as described above will be referred to as a base station or a Radio Base Station (RBS). A user equipment as described above, will in this disclosure be referred to as a user equipment or a UE.
The expression DownLink (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
The base stations in the cellular network are generally not time synchronized. This results in a timing offset difference between the base stations, which may also be referred to as a “timing relation” between the base stations.
Accurate timing relation information of base stations may be necessary for several purposes, for example for positioning of a user equipment.
Positioning of a user equipment in the cellular network may be based on measurements of the time of flight, i.e. of the time it takes for a radio signal to move between the user equipment and several respective base stations in its vicinity, or measurements of the differences between the times of flight between the user equipment and the respective base stations, e.g. measurements of the so called Time Difference Of Arrival (TDOA).
The time of flight of radio signals may be converted to an absolute distance by multiplying with the speed of light. If the times of flight between the user equipment and at least three base stations with known positions, or the differences of the respective times of flight, are measured, the user equipment's position may be estimated by so called trilateration, or multilateration, techniques. This will be described further down in this document.
Positioning methods using this principle include Enhanced Observed Time Difference (E-OTD), Uplink Time Difference of Arrival (UTDOA) and Observed Time Difference of Arrival (OTDOA). These methods are very similar in terms of their requirements on timing accuracy, and E-OTD is actually the 2G version OTDOA. All the above positioning methods have strict requirements on base station clock accuracy, and the timing offset differences of the involved base stations are parameters to be used during position calculation.
A problem is that such timing relation information may be either hard to obtain or may not be provided with good reliability due to implementation reasons.
One way to estimate the timing offset of a base station, is to compare the base station clock to a synchronization pulse from a Global Navigation System Satellite (GNSS), which may be received by a so called GNSS receiver.
A problem is that it is hard for the base station to achieve a timing accuracy better than about 100 ns. Even with a GNSS receiver, the 1 pps (pulse per second) signal from GNSS has limited accuracy, e.g. 100 ns, and is subject to some other factors such as the physical distance between the GNSS receiver and the base station.
For positioning purposes, the synchronization may need to be done to a level of accuracy of the order of tens of nanoseconds, as 10 nanoseconds uncertainty contributes 3 meters error in the position estimate.
Moreover, even after synchronization of base stations, drift and jitter in the synchronization timing must also be well-controlled as these also contribute to uncertainty in the position estimate.
Information about timing relations between base stations in the cellular network, i. e. the timing offset differences between the base stations, may also be required for other purposes than for positioning of user equipments. For example in LTE-TDD, Multicast Broadcast Single Frequency Network (MBSFN), Coordinated MultiPoint (CoMP), and enhanced Inter Cell Interference Cancellation (ICIC) in 3GPP specifications.
A problem is that sometimes the solution may be costly and subject to environment change. For example, a GNSS receiver may be required, and this normally may not function properly indoors.